The Mechanism Of Spontaneous Electrical Activity And Hyperalgesia Of Myofascial Trigger Point

BackgroundMyofascial trigger points (MTrPs), defined as hyperirritable nodules of spot tenderness in a muscle taut band, have been suggested to be the common cause of myofascial pain syndromes (MPS) which is one of the most prevalent and disabling pain syndromes in chronic pain patients. A latent MTrP is clinically quiescent with respect to spontaneous pain, but is painful when stimulated manually or with a needle. It does everything else that an active trigger point does, but does not reproduce familiar pain.To date, the pathogenesis of myofascial trigger points remains elusive. Snapping palpation and/or dry needling of the myofascial trigger points can evoke local and referred pain in addition to a local twitch response. Spontaneous electromyography activity (SEA), as measured by needle electrodes at myofascial trigger points, is a common characteristic; however the source of the SEA and the relationship between pain and SEA are unknown and the role of sensory afferent fibers and motor units at myofascial trigger points has not been fully clarified.An increased H-reflex response induced by intramuscular electrical stimulation at MTrPs has also been reported, indicating the hypersensitive large diameter muscle afferents at MTrPs. Human study found that electrically induced muscle pain was decreased after ischemic compression block (ICB) for 20min when it mainly affects A muscle afferents. The fatigable muscle fiber may increase large diameter myelinated muscle afferents input. Thus, it is possible that large diameter muscle afferents may play a role in the pathophysiology of MTrPs.We hypothesize that active myofascial trigger points are a result of hypersensitive A muscle spindle afferents. To validate the hypothesis, we did below experimentsAimThe aim of this present study is to test the hypothesis that myofascial trigger points are a result of hypersensitive A muscle afferents fiber and these hypersensitive A muscle afferents fiber contribute to the pathophysiology of myofascial trigger points.MethodsForty-six male Wistar rats were used in this study which consisted of three sessions. A specially designed hollow-needle electrode was used for searching MTrP in the biceps femoris muscle of anesthetized rat which SEA will be recorded for 3 min.In the first session, in 12 rats, the SEA is recorded before and after dissection of ventral or dorsal roots innervating biceps femoris muscle. The average amplitude and frequency of the intermittent spike activity of SEA were analysised. The average amplitude and frequency of the intermittent spike activity of EMG were considered to be significant change when lower or higher 50% than before.In the second session, SEA recorded at myofascial trigger points was compared before and 10min after a sensory nerve block by TTX injection or injection of iso-osmia physiologic saline at sciatic nerve in 24 rats.In the third session, the threshold of mechanical stimulation that elicited the ipsilateral hindpaw shaking behavior was assessed by electronic von frey before and 5mn,15min and 30min after a local sensory nerve block by TTX at the region of MTrP and at non-MTrP in 10 rats.ResultsThe average amplitude and frequency of the intermittent spike activity of SEA were considered to be significant change when lower or higher 50% than before.1. In the experiment of dissection of dorsal roots, there were 7 rats in which SEA evoked from MTrP were significantly decreased and 5 rats being no significantly change. In the experiment of dissection of ventral roots, there were only 1 rats in which SEA evoked from MTrP was significantly decreased and 11 rats being no significantly change.Chi-square test revealed a significant difference in the incidence of SEA after dissection of ventral or dorsal roots (Chi-square value:6.314, P< 0.05).2. In this experiment, there were 7 rats in which SEA evoked from MTrP were significantly decreased after A sensory nerve block by TTX and 2 rats decreased after injection of iso-osmia physiologic saline at sciatic nerve. Chi-square test revealed a significant difference in the incidence of SEA after A sensory nerve block (Chi-square value:4.444, P< 0.05). 3. Two-way AN OVA revealed that there were significant difference in mechanical threshold for evoking the hind paw responses between MTrP region and non-MTrP region at the time of before and 5,15,30min after TTX local block with significant interaction between these two factors (F=12.141, P<0.01; F=96.128, P <0.01; F=6.913, P<0.01). Mechanical threshold was significantly lower in MTrP than in non-MTrP at pre-block by TTX (q=12.438, P< 0.01). Mechanical thresholds at 5,15,30min after TTX local block were significantly higher than before at MTrP region (q=4.476, P<0.01; q=7.504, P<0.01; q=9.313, P<0.01). Mechanical threshold at 30min after TTX local block at MTrP region was significantly lower than that of non-MTrP region (q=4.255, P<0.01). There was also significantly lower in Mechanical threshold at 5min after TTX local block at non-MTrP region than before (q=3.569, P<0.05). No significant difference was found at before-.15min and 30min after TTX local block at non-MTrP region(q=1.129, P> 0.05; q=0.547, P > 0.05).Conclusion1. These results suggest that A muscle afferent fiber play an important role in myofascial pain and in the SEA of MTrP.2. The excitability of cornu posterius medullae spinalis may contribute the SEA of MTrP.